Ensure that Form.fill doesn't attempt to validate

Fixes #1264
Don't send empty chunks

An empty chunk means the end of a chunked transfer encoding stream, so
the chunking enumeratee should filter them out and not send them.

Fixes #2175
Merge pull request #1264 from jroper/1264-no-validate-on-fill

Form.fill still calls validation function
Merge pull request #2175 from jroper/2175-empty-chunks

Ok.chunked is  *not* ignoring empty blocks
Remove 1 error class